Now, and this opinion may depend on whether you're claustrophobic like me or not, but that's a pretty frightening concept in itself. And one that can easily be screwed up in the wrong hands. Does The Descent succeed? Yes and no.
The yes applies mostly to the aforementioned first 45 or so minutes. The slow, pleasant beginning (save for the bit with Sarah losing her husband and daughter in the car crash, but it reverts back to its previous atmosphere soon after) along with the characters taking everything lightly, dully ticking off the warnings for caving as they hiked to their destination and overall just behaving like normal, everyday people made for a great distraction and contrast to the horrific events that were to come. When they got into the cavern, I was convinced this was going to be one of my top horror films of all time. The crawls in the tunnels were wonderfully claustrophobic.
Then, after 45 or so minutes pass, they get to what is intended to be the "true" horror of the film and, well...I won't say the film "failed" at this point as, for what it's worth, it was still a good movie--a great one in comparison to the majority of the horror films released nowadays. But I still have more than a few bones to pick with it that prevent me from wholeheartedly finding it an excellent film, much less a scary one. And the biggest one is the monsters. The main, selling "scare" of the film.
I didn't mind the quick, unclear glimpses we got of them early on in the film, before they discovered the group. I didn't mind the clear glimpse of the one we got on the video camera in night vision either--in fact, that was great. Made me jump in my seat. Then their appearances became prevalent to the point of overabundant and then, well, then they just stopped being scary. They became an old joke that was being told over and over again. But my biggest frustration, I think, was how unnecessary they were when it came down to it. The film didn't need any scary monsters because the premise of being trapped and lost in an unknown cavern was scary enough as is. It could carry its own weight with delivering the scares. The fact that the focus shifted from the sheer claustrophobia of the setting and the horror of being lost in it to the characters fending off against monsters was a tragically wasted opportunity, in my opinion.
I've heard that there's a chance that the monsters may be a figment of Sarah's imagination, which brings me to my next bone to pick with the film: the horribly tacked on ambiguity. I'm a big fan of ambiguity, especially when applied to horror--I think it's a great way of increasing the "fear of the unknown" factor, if that makes sense. Here, I wasn't feeling it. The scenario of Sarah killing Juno and successfully escaping the cavern, followed by her waking up and hallucinating her daughter in front of her felt more like a last minute attempt to make the film appear "deep" than an honest extra dimension added to the film's story and protagonist. Adds a layer to the title (the "descent" into madness), maybe. But not to the film itself.
And that about sums up my main grievances with it. Would I recommend this film? Sure. The peeves I had were only that of my own, so your mileage may vary on the path the film goes down.